The present invention relates in general to precision power supplies and in particular to a new and useful low power, high efficiency switching power supply which is particularly useful for powering a 4 to 20 mA current loop.
Normally, low power supply is of less than 50 mW, with split voltage sources (+ voltages) are accomplished by establishing a floating common (elevated) or by a low efficiency switching power supply. It is imperative however that a power supply operating from a 4-20 mA current loop be highly efficient to maximize circuit functions.
Two-wire analog transmission systems are well known. Such systems include a transmitter which is connected to a power supply by two wires which form a current loop. The transmitter includes, as at least one of its features, a transducer which senses a condition such as pressure or temperature. This condition is know as a process variable (PV).
A power supply is connected to the two wires to close the current loop. It is also conventional to provide a resistor in the current loop. The transmitter amplifies the signal from its transducer and this amplified signal is used to draw a certain current from the power supply which is proportional or otherwise related to the process variable. It is conventional to draw from a minimum of 4 (mA) to a maximum of 20 mA. The current between 4 and 20 mA passes through the resistor to produce a voltage drop across the resistor. This voltage drop can be measured to give a value for the process variable.
It is noted that the 4 mA minimum current is required to energize the circuitry of the transmitter. Any excess current above this 4 mA level is taken as a value which can be used to determine the process variable.
It is known that such 4-20 mA two-wire systems have an accuracy which is limited to around 0.1% at best. These systems are also essentially unidirectional with the transmitter being essentially uncontrolled and transmitting continuously.